The present invention relates to abrasive particles, particularly cubic boron nitride (CBN) and diamond, and more particularly to vitreous bonding grinding wheels containing such abrasive particles.
In U.S. Pat. No. 4,951,427, abrasive particles are taught to be coated with at least 20% by weight of a refractory metal oxide selected from the group consisting of titania, zirconia, aluminum, and silica. Such coated abrasive particles are suitable for embedding in a bonding matrix. The coating protects the abrasive particles, primarily CBN, from being attacked by fluxes commonly used in vitreous bond systems. Such alkali metal oxide fluxes attack CBN to form alkali borates with concomitant evolution of large quantities of gas, primarily nitrogen and some ammonia if water is present. These phenomena lead to bloating and slumping of the desired grinding wheel form.
Other propsals include U.S. Pat. No. Re. 31,883 which discloses CBN particles can be coated with from 30 to 80 wt % of nickel and are useful in the manufacture of resin bonded grinding wheels. U.S. Pat. No. 4,011,064 described CBN particles having a rough, flakey granular covering consisting essentially of a coating ranging in composition from metal to a mixture or boride and nitride of the metal intermediate the surface coating and the CBN particles. U.S. Pat. No. 4,399,167 proposes a method of metal coating diamond and CBN. U.S. Pat. No. 3,528,788 discloses resin bonded grinding wheels having embedded therein diamond grit material coated with from 25 to 275 parts of metal per 100 parts of diamond, where the metal is selected from a defined list.
It is also known in the art that the retention of diamonds of the metal bond or saw type in metal matrices can be improved by providing the particles with a double layer coating, such as disclosed in U.S. Pat. Nos. 3,826,630 and 3,929,432.
In order to maximize performance in a grinding wheel, it is important to distribute the abrasive grain very uniformly and, in the case of vitreous bonded wheels, to achieve the correct porosity uniformly and consistently. This latter goal is not easily achieved using present techniques. Control of porosity speaks to several factors in performance, including for example, the crushability of the wheel and the coolant liquid retention.